The present invention relates to a vehicle behavior control apparatus capable of calculating accurate target yaw rates used for various vehicle behavior control means.
In recent years, various vehicle behavior control means such as a limited differential control, a driving force distribution control and the like have been applied to actual vehicles in market. In a vehicle behavior control means, control variables are calculated based on target yaw rates established according to vehicle motion conditions.
Generally, in vehicle behavior controls, if the same target yaw rate as that on a road surface with high friction coefficient is established when a vehicle travels on a road surface with low friction coefficient, a target yaw rate is established to a yaw rate exceeding the limit of yaw rate under which a vehicle can travel safely on a road surface with low friction coefficient and as a result the yaw rate conversely strengthens a tendency of spin of the vehicle.
For example, in case of a front and rear wheels driving force distribution control of a four wheel drive vehicle, when a yaw rate actually detected (actual yaw rate) is smaller than a target yaw rate, it is necessary to correct a driving force distribution of rear wheels in an increasing direction in order to enhance a turning ability. Therefore, in case where the same target yaw rate as that on a road surface with high friction coefficient has been established during traveling on a road surface with low friction coefficient, the front and rear wheel driving force distribution control tries to realize a target yaw rate exceeding a yaw rate under which a vehicle can travels in a stable manner. As a result, the driving force of rear wheels increases excessively, this leading to a spoilage of the traveling stability of the vehicle.
In addition to this, since the yaw response speed versus to steering also goes down on a road surface with low friction coefficient (hereinafter, referred to as xe2x80x9clow friction coefficient roadxe2x80x9d), when the steering wheel is cut back or in other case, the discrepancy between an actual yaw rate and a target yaw rate further increases and as a result the tendency of spin glows stronger.
To solve this problem, Japanese Patent Application Laid-open No. Toku-Kai-Hei 5-278489 discloses a technique in which the target yaw rate is corrected in a smaller direction, as detecting values of friction coefficients of road surfaces decrease.
On the other hand, in case where the accuracy in establishing the target yaw rate is low, since control gains or control sensitivities based on the deviation of actual yaw rates from target yaw rates must be restricted from the view point of preventing wrong way operations, it becomes difficult to fulfill adequate disturbance stability. As the causes of lowering the accuracy in establishing the target yaw rate, changes of road friction coefficients, an effect of responsibility when the steering wheel is rapidly operated, asymmetric property of the vehicle, nonlinearity when the steering wheel makes a large turn, changes of laden conditions and the like, are considered.
Hence, Japanese Patent Application Laid-open No. Toku-Kai-Hei 6-72169 discloses a technology in which a dynamic characteristic of the vehicle with respect to steering operation is enhanced by establishing a steering responsibility of the vehicle, specifically, a first order lag time constant of yaw response of the vehicle with respect to steering operation according to vehicle speeds.
However, the former prior art, Toku-Kai-Hei 5-278489, has a problem with responsibility and accuracy when a friction coefficient on road surface (hereinafter referred to as a road friction coefficient) is detected. Further, the prior art has a restriction in detectable vehicle traveling conditions and also a problem of difficulty in optimization of target yaw rates. Further, the latter prior art, Toku-Kai-Hei 6-72169, has an inadquate optimization of target yaw rate because the steering responsibility of the vehicle varies according to not only the vehicle speed but also the steering velocity.
It is an object of the present invention to provide a vehicle behavior control apparatus capable of accurately optimizing and establishing target yaw rates under various conditions including ones being suffered from various disturbances.
To attain the object, the vehicle behavior control apparatus a target yaw rate establishing means of the vehicle behavior control apparatus comprises a vehicle mass estimating means for estimating a vehicle mass based on a vehicle acceleration or deceleration, a mass distribution calculating means for calculating a mass distribution between front and rear axles based on the vehicle mass, a front and rear axles to gravity center distance calculating means for calculating distances between front and rear axles and a center of gravity, a front wheel steering angle calculating means for calculating a steering angle of a front wheel based on a steering wheel rotation angle, a stability factor calculating means for calculating a stability factor based on the vehicle mass, the vehicle mass distribution, the distances between front and rear axles and center of gravity and the steering angle of the front wheel and a target yaw rate calculating means for calculating the target yaw rate based on the stability factor.
Further, the front wheel steering angle calculating means includes a steering gear ratio varying means capable of varying a steering gear ratio according to a steering wheel rotation angle. The target yaw rate establishing means calculates the target yaw rate based on a steering gear ratio established by the steering gear ratio varying means.
Further, the target yaw rate establishing means includes a road friction coefficient estimating means for estimating a road friction coefficient. The target yaw rate establishing means calculates the target yaw rate by estimating a lag of target yaw rate with respect to steering according to the estimated road friction coefficient.
Further, the target yaw rate establishing means includes a warning means for warning the road friction coefficient and calculates the target yaw rate by estimating a lag of target yaw rate with respect to steering according to the estimated road friction coefficient and warns the estimated road friction coefficient by the warning means.
Further, the target yaw rate establishing means has a reference yaw rate calculating means for calculating a reference yaw rate which is a target yaw rate calculated by estimating a lag of yaw rate with respect to steering and the reference yaw rate is corrected by the reference yaw rate multiplied by a target yaw rate correction factor to obtain a final target yaw rate.
Further, the target yaw rate establishing means includes a road friction coefficient estimating means for estimating a road friction coefficient and calculates the target yaw rate by estimating a lag of target yaw rate with respect to steering according to the estimated road friction coefficient.